Small violations of unitarity, the phase in B-s(0) - (B)over-bar(s)(0) and visible t -> cZ decays at the LHC

We show that it is possible to accommodate the observed size of the phase in Bs0-[overline B]s0 mixing in the framework of a model with violation of 3×3 unitarity. This violation is associated to the presence of a new Q=2/3 isosinglet quark T, which mixes both with t and c and has a mass not exceeding 500 GeV. The crucial point is the fact that this framework allows for chi[equivalent]arg(-VtsVcbVtb*Vcs*) of order lambda, to be contrasted with the situation in the standard model, where chi is constrained to be of order lambda2. We point out that this scenario implies rare top decays t-->cZ at a rate observable at the LHC and |Vtb| significantly different from unity. In this framework, one may also account for the observed size of D0-[overline D]0 mixing without having to invoke long-distance contributions. It is also shown that in the present scenario, the observed size of D0-[overline D]0 mixing constrains chi[prime][equivalent]arg(-VcdVusVcs*Vud*) to be of order lambda4, which is significantly smaller than what is allowed in generic models with violations of 3×3 unitarity.